Charmonium Spectroscopy From Fermilab E835

Charmonium Spectroscopy FromFermilab E835Todd K. Pedlar 1Department of Physics and AstronomyNorthwestern UniversityAbstract. Fermilab E835 is dedicated to the study of charmonium spectroscopy viaresonant formation in �pp annihilations. This paper presents preliminary results oncharmonium spectroscopy from E835 data taken in the 1996-7 Fixed Target runningperiod. Speci�cally, results for the resonance parameters of �c (11S0 ), the partialwidth of �2 (13P2 ) and a search for �0c (21S0 ) are presented, as is the �rst evidencefor �0 (13P0 ) formation in �pp annihilation.INTRODUCTIONPrecision spectroscopy of the charmonium system can yield quantitative resultsconcerning the nature of QCD in the non-perturbative regime, where it is not wellunderstood. The mass of the charm quark is small enough that c�c cross sections arereasonably large, yet it is large enough that its motion within bound charmoniumis nearly non-relativistic. The spectrum of charmonium states is shown in Fig. 1.From the discovery of J= in 1974 until the mid-1980's, nearly all charmoniumspectroscopy was studied via e+e� annihilation at facilities like SLAC and DESY.Because e+e� annihilation proceeds via a virtual photon, only vector JPC = 1��states may be formed directly in this manner. The study of non-vector states (suchas the �J states) is possible only through the observation of the radiative decays 0 (or J= )! (c�c ) + . Thus the mass and width measurements for these statesare limited by the photon energy resolution of the detector system. While mostof the charmonium states were discovered in such experiments, their masses, and1) on behalf of the Fermilab E835 Collaboration: G. Garzoglio, K. E. Gollwitzer, A. Hahn,W. Marsh, J. Peoples Jr., S. Pordes, J. Streets, S. Werkema (Fermi National Accelerator Labo-ratory), M. Ambrogiani, W. Baldini, D. Bettoni, R. Calabrese, P. Dalpiaz, E. Luppi, R. Mussa,M. Savri�e, G. Stancari (INFN and University of Ferrara), A. Buzzo, M. Lo Vetere, M. Macr��,M. Marinelli, M. Pallavicini, C. Patrignani, E. Robutti, A. Santroni (INFN and University ofGenova), G. Lasio, M. Mandelkern, J. Schultz, M. Stancari, G. Zioulas (University of Califor-nia at Irvine), X. Fan, S. Jin, J. Kasper, P. Maas, T. K. Pedlar, J. L. Rosen, K. K. Seth,A. Tomaradze(Northwestern University), S. Bagnasco, G. Borreani, R. Cester, F. Marchetto,E. Menichetti, N. Pastrone, P. Rumerio (INFN and University of Torino) and R. McTaggart1.

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FIGURE 1. The spectrum of charmonium states.especially widths, were not well measured. In order to do precision spectroscopyof charmonium, a new technique was required. Such a technique, charmonium via�pp annihilation, was pioneered in 1984 by R704 at CERN [2]. Fermilab E760 andE835 have fully exploited this technique, and have obtained results which representimprovements in precision, in some cases by orders of magnitude.The chief advantage of the technique of charmonium formation via �pp annihi-lation is that all charmonium states may be formed directly - the �pp annihilationprocess occurs through two or three gluons in the intermediate state, which mayhave any value of JPC . Since every charmonium state can be formed directly, itsresonance parameters may be determined with a precision dependent only on theknowledge of the beam energy and energy distribution, and event statistics.FERMILAB EXPERIMENT E835The E835 experiment is located in the Fermilab Antiproton Accumulator (SeeFig. 2). The circulating beam of antiprotons intersects a hydrogen gas jet target toproduce �pp interactions. During Fermilab �xed target operations, the Accumulatorstores antiprotons at 8.9 GeV, and decelerates them to the momentum required forthe resonant formation of a c�c state (antiproton momenta of 4 to 7 GeV, forcharmonium masses from 3 to 4 GeV).The stochastically-cooled �p beam consists of up to � 1012 �p which orbit the474:0454 m path length of the accumulator with a frequency of � 0:62 MHz. Thisfrequency is measured using the Schottky noise spectrum of the beam to about2 parts in 107. The beam intersects the gas jet target [3], whose density in the

- TARGET pFermilab Antiproton Source

BOOSTER

DEBUNCHER

MAIN RING

E835

ACCUMULATOR

FIGURE 2.interaction region can be varied between 1012 and 1014 atoms/cm3, in order to keepthe instantaneous luminosity nearly constant (typically, 2� 1031cm�2 sec�1 ).The center of mass energy ps is determined by the revolution frequency andthe orbit length, which is measured accurate to �0:7 mm by a calibration scan atthe 0 , whose mass is 3686:00 � 0:1 MeV. [4] Deviations of the beam from thisreference orbit length are measured by beam position monitors to �1 mm, so thatps may be known to � �50 keV. The beam momentum spread, which is directlyrelated to the width of the Schottky noise spectrum, is �p=p ' 1� 2� 10�4. This�p corresponds at the 0 to a center of mass energy spread �(ps) � 200 keV.The E835 detector (See Fig. 3) is an upgrade of the E760 detector system, whichhas been described in several articles. [5]- [8] It is an azimuthally symmetric non-magnetic spectrometer with full azimuthal acceptance, and polar angle � coveragefrom 2� to 70� . The main components of the detector are a 1280-element leadglass EM calorimeter, a large-acceptance segmented threshold �Cerenkov counter,and several inner tracking detectors. Luminosity is measured to � �2:5% by threesolid-state detectors which observe elastically scattered protons at � � 86:5� . [8]In E835, charmonium states are studied by decelerating the �p beam so thatps isvaried in small steps across the resonance under study. At each step, the yield of the�nal state particles from the electromagnetic decays such as e+e�, J= ! e+e� or is measured. The number of events divided by luminosity taken at eachpoint, plotted versus the center of mass energy, gives the resonance excitationcurve. This is just the convolution of a Breit-Wigner resonance cross section (whichis proportional to the product of the branching ratios for the formation and thedecay of the resonance, Bin � Bout) and the measured beam energy distributionfunction. Fig. 4 shows an example of such an excitation curve from the scan of �2in E760. [9]

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FIGURE 3. Schematic of the E835 detector system.PRELIMINARY RESULTSE835 collected an integrated luminosity of approximately 145 pb�1, resulting inapproximately 5 billion triggered events written to tape. From these data, we haveobtained the following results (of which the �rst four are discussed here):� Measurements of �c resonance parameters M , � and �(�c ! )� Measurement of the partial width �(�2 ! )� Search for �0c ! in the region ps = 3575 to 3660 MeV� First observation of �0 in �pp annihilation� Measurements of{ 0 ! e+e� and 0 ! J= X branching ratios{ �2 , �1 radiative decay angular distributions{ Proton electromagnetic form factors{ J= ! �pp and 0 ! �pp angular distributionsDetection of (c�c ) via �pp ! c�c ! Three of the four states which we address in this article are observed in E835 intheir decay into two photons. These C-even states are di�cult to study in �pp colli-sions, because their branching fractions are typically of the order of 10�4 (andcross sections for �pp ! (c�c )! of tens of picobarns). Furthermore, the limitedacceptance of the E835 detector results in large backgrounds from proli�c (� 100�b)all-neutral processes such as �pp ! �0 and �pp ! �0 �0. These backgrounds arise

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FIGURE 4. Example of a scan of a c�c state. The solid curve is a �t to the acceptance ande�ciency-corrected data, while the dotted curve shows the beam energy spread. (From Ref. [9])when either one or both �0 decay asymmetrically, and the low-energy is notdetected, either falling below threshold or being outside the detector acceptance.This results in typical signal-to-background ratios of 1:1 to 1:3.The partial widths of charmonium states are of great theoretical interest. Thec�c ! process is simply calculable to zeroth order in QED. However, there arelarge (up to 75%) �rst-order QCD radiative corrections [10]. Unfortunately, mostexisting c�c ! measurements have large errors; hence the present measurements.In E835 events are selected with a topological trigger designed for e�cientdetection of a high-mass pair of electromagnetic clusters in the CCAL, with nocharged tracks identi�ed in the tracking hodoscopes. In order to minimize theproblem of pile-up, each channel of the CCAL is instrumented with TDCs, toreject out-of-time interactions which contaminate our event sample due to runningat high luminosity. Of the neutral two-body trigger sample, events were acceptedwhich contained only two on-time CCAL clusters, and which had no additionalclusters which paired with one of the on-time clusters to form an invariant massconsistent with m �0 . A 4C kinematical �t was made on these events, and a �2probability � 5% was required. Further, a cut on the center of mass angle �� wasmade in order to reduce the background contribution due to misidenti�cation of�0 �0 and �0 events, and to thus increase the signal-to-background ratio.Measurement of �c (11S0 ) Resonance ParametersThe observed cross section for the scan in the �c mass region for jcos(��)j � 0:20is shown in Fig. 5 plotted as a function of the center of mass energy, ps. Thedata have been �t to a Breit-Wigner resonance and a power-law background ofform �BKG = A � (2984:8=ps)B. The results of this �t are shown in Table 1compared to those in PDG98 [4]. We note that compared to the PDG98 values for

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FIGURE 5. Observed cross section for �pp ! for j cos ��j � 0:2 in the �c region.TABLE 1. Measurements of �c resonance parameters.Parameter PDG98 E835Mass 2979:8� 2:1 MeV 2984:8� 1:9 MeV� 13:2+3:8�3:2 MeV 17:8+7:2�5:9 MeVB(�pp ! �c )� �(�c ! ) - 4:4+1:8�1:6 eV�(�c ! ) 7:5+1:6�1:4 keV 3:7+1:5�1:3 � 1:2 keV�c , our mass is � 5 MeV larger, the width is � 5 MeV larger, though with largeruncertainty, and our two photon width is nearly a factor two smaller.Search for �0c ! �0c , the radial excitation of �c , has been reported in only one experiment, CrystalBall, in 1982, with a mass of M(�0c ) = 3594� 4 MeV. [12] E760 searched for �0c at6 data points from 3591 to 3621 MeV, and obtained no conclusive result. E835 hassearched in a much wider mass region (3575 to 3660 MeV), investing � 30 pb�1. Wehave also failed to �nd any evidence for �0c excitation. From these data we can onlyderive an upper limit for the branching fraction product B(�pp ! �0c )�B(�0c ! )of 9 � 10�8 at 95% con�dence for an assumed �0c width of 5 MeV. The observed cross sections for j cos ��j � 0:4 are plotted from 3570 to 3700 MeV in Fig. 6.

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FIGURE 6. Observed cross sections for �pp ! for j cos ��j � 0:4 in the �0c search region.The dashed curve represents the expected signal atM(�0c ) = 3495 MeV for �(�0c ) = 5 MeV, andB( )�B(�pp ) equal to those of �c .Measurement of �(�2 ! )E760 reported a measurement of �(�2 ! ) = 0:321�0:078�0:054 [13] whichdi�ered signi�cantly from values obtained in other experiments, such as CLEO. [14]We have made a new measurement of the same quantity which con�rms the E760value, and has smaller errors. We obtain �(�2 ! ) = 0:311�0:041�0:031 keV.The data for this measurement are shown in Fig. 7.

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FIGURE 7. Observed cross sections for �pp ! for j cos ��j � 0:4 in the �2 region.

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FIGURE 8. Measurements of the running coupling constant �S at various values of Q2.�S from charmonium decaysFrank Wilczek has emphasized the need for measurements of �S at small valuesof Q2. [15] Our data for �c ! and �2 ! give us two separate ways ofcalculating �S at Q2 = (mc)2, using the pQCD formulae with �rst order radiativecorrections: [10]�(�c ! gg)�(�c ! ) = �TOT (�c )�(�c ! ) = 9�S 28�2 � (1 + 4:8�S =�)(1� 3:4�S =�) (1)�(�2 ! gg)�(�2 ! ) = �(�2 )� �(�2 ! J= )�(�2 ! ) = 9�S 28�2 � (1� 2:2�S =�)(1� 16�S =3�) : (2)Using our measured values for �TOT (�c ) and �(�c ! ), we calculate �S (mc) =0:33 � 0:05. Using the PDG98 values of �TOT (�2 ) = 2:00 � 0:18 MeV andB(�2 ! J= ) = 0:135 � 0:011, and our measured value of �(�2 ! gg), we�nd �S (mc) = 0:36 � 0:02. The two results are consistent, giving an averageof �S (mc) = 0:35 � 0:02. This result is in excellent agreement with �S (m� ) =0:35 � 0:03. [4] For comparison with other data on �S , our value of �S (mc) isplotted along with the compilation from PDG98 [4] in Fig. 8.Detection of �0 via �pp ! c�c ! J= The extremely background-free channels for the investigation of c�c states inE835 are those which include a single high-mass e+e� pair, such as c�c ! e+e� orc�c ! J= ! (e+e�) . These events are triggered by a high mass pair of clustersin the CCAL, associated with two tracks in the inner charged tracking detectorsand at least one associated signal in the �Cerenkov. This trigger was successfullyused in E760 to measure the resonance parameters of �1 and �2 . The �0 statewas not investigated.A preliminary study of �0 was performed with limited luminosity at the endof the 1997 run of E835, a very clean J= signal in the region was observed.

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FIGURE 9. Cross section for �pp ! J= in the �0 region.TABLE 2. Measurements of �0 resonance parameters.Parameter PDG98 E835Mass 3417:3� 2:8 MeV 3415+2:1�1:7 MeV� 13:5� 5:3 MeV 13:9+5:3�3:9 MeVB(�pp ! �0 )�B(�0 ! J= ) - 2:80+0:63�0:46 � 10�6B(�0 ! ) � 9� 10�4 4:24+0:96�0:70 � 1:16� 10�4This represents the �rst measurement of the branching fraction B(�0 ! �pp ). Thecross section for this reaction appears in Fig. 9, and the resonance parametersderived from the maximum likelihood �t to the data are found in Table 2. PDG98gives only an upper limit of B(�0 ! �pp ) � 9 � 10�4, while we have obtainedB(�0 ! �pp ) = (4:24+0:96�0:70 � 1:16)� 10�4.FUTURE PLANS FOR E835The Fermilab PAC has approved E835 for running again in 1999. Since severalof the stated priorities of E835 were not achieved during the 1996-97 �xed targetrunning period, these measurements are the �rst scheduled for the extension run.These are, in order of priority,� Con�rm the 1P1 discovery by E760 and measure its resonance parameters.� Measure �0 resonance parameters with better precision.� Make a new search for �0c .� Scan above DD threshold for 3D2 , 1D2 states.

CONCLUSIONSE760 was the �rst experiment to fully exploit the technique of charmonium for-mation in �pp annihilation. It made several contributions to the �eld of precisioncharmonium spectroscopy. E835 has advanced these e�orts further, and has ob-tained new results. We look forward to running again in 1999, and completing ourprogram of study of the charmonium spectrum.ACKNOWLEDGEMENTSThe E835 Collaboration would especially like to thank our colleagues in FermilabBeams Division, without whose e�orts this experiment would not have been possi-ble. We also acknowledge the support of the U.S. Department of Energy (DOE),the U.S. National Science Foundation (NSF), and the Italian Istituto Nazionalede Fisica Nucleare (INFN). The speaker wishes to thank the organizers of thisworkshop for their hard work in putting together an excellent conference.REFERENCES1. Pennsylvania State University.2. C. Baglin, et al. , Nucl. Phys. B286 (1987) 5923. D. Allspach et al. , Nucl. Instr. Meth. A410 (1998) 195.4. C. Caso et al. (Particle Data Group) European Physical Journal C3 (1998) 1.5. T. A. Armstrong, et al. , Nucl. Phys. B373 (1992) 35.6. L. Bartosek et al. , Nucl. Instr. Meth. A301 (1991) 47.7. C. Biino et al. , Nucl. Instr. Meth. A317 (1992) 135.8. S. Trokenheim et al. , Nucl. Instr. Meth. A355 (1995) 308.9. T. A. Armstrong et al. , Phys. Rev. Lett. 68 (1992) 1468.10. W. Kwong et al. , Phys. Rev. D37 (1988) 3210.11. T. A. Armstrong et al. ,Phys. Rev. D52 (1995) 4839.12. C. Edwards et al. , Phys. Rev. Lett. 48 (1982) 70.13. T. A. Armstrong et al. , Phys. Rev. Lett. 70 (1993) 2988.14. V. Savinov and R. Fulton, (CLEO Collaboration) Proc. 10th Int. Workshop onPhoton-Photon Collisions, She�eld, England, 1995, ed. by D. J. Miller et al. , (WorldScienti�c, 1995) p. 203, J. Dominick et al. , (CLEO Collaboration), Phys. Rev. D50(1994) 4265.15. F. Wilczek, Proc. Int. Symp. on Lepton and Photon Interactions, Ithaca, NY, 1993,ed. by P. Drell and D. Rubin, AIP Conf. Proc. 302 (1994) 593.

Charmonium Spectroscopy From Fermilab E835

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